This invention relates to an apparatus for magnetically recording a pulse signal and, more particularly, to such apparatus wherein the pulse signal is a suppressed-carrier balanced modulated signal which, upon recording and reproducing, exhibits an improved error characteristic.
In many applications, information, such as audio information, alpha-numeric information, and the like, is represented by pulse signals, and these pulse signals are recorded on a magnetic recording medium. For example, it is known that a video tape recorder (VTR) can be used to record such information-representing pulse signals on magnetic videotape. Typically, the information is encoded as a pulse coded signal, such as by pulse code modulation (PCM), and this pulse coded signal is recorded.
Various techniques are used to record pulse signals on magnetic tape. In one technique, a carrier is frequency modulated with the pulse signal, and then this FM signal is recorded. In another type of recording technique, the pulse signals are recorded directly by the so-called direct saturation method. This direct saturation method is used primarily in mass storage systems, such as digital storage systems.
When either the FM recording or direct saturation recording techniques are used, modulation noise has a deleterious influence on the reproduced signals. Also, the error characteristic of the recorded and reproduced signals is deteriorated if the signal-to-noise (S/N) ratio deteriorates. On reproducing the FM or direct saturation pulse signals, the error rate in the reproduced signals may be undesirably high.
In one method of reproducing a pulse signal, the rising and falling (or leading and trailing) edges of each recorded pulse are reproduced, generally as differentiated pulses. The presence of such differentiated pulses may be detected by comparing the reproduced signal to one or more predetermined reference levels. When these reference levels are traversed by the differentiated pulses associated with the pulse signal (i.e. the rising and falling edges), the original leading and trailing edges of that pulse signal can be recovered. Then, from these recovered leading and trailing edges, the original pulse signal can be reproduced. However, it is known that, in many tape recording systems, such as in many VTR's there is an inherent distortion factor associated with higher frequencies, these higher frequencies typically being within the range encompassed by the third harmonic of the pulse coded signal. Because of this distortion factor, the differentiated pulses which represent the rising and falling edges of the pulse signal appear to be shifted in the time axis (or time base) direction. Such a time shift erroneously interferes with the sensing and reproduction of the original pulse signal. That is, because of this time shift, a differentiated rising edge pulse may be detected at a time location whereat the original pulse signal exhibits a falling edge. Hence, the resultant sensing of the original pulse signal will be incorrect. Thus, the error characteristic in reproducing pulse coded signals suffers deterioration because of this third harmonic distortion factor.